The invention relates to methods for measuring the absolute speed of a moving body relative to the ground, and to apparatuses for implementing said methods. It may be applicable to automobiles or trains.
Driving assistance and safety systems require knowledge not only of the speed of rotation of the wheels, but also of the absolute speed of the vehicle relative to the ground.
When a vehicle encounters a sheet of ice for example, and if the driver brakes suddenly, the wheels lock owing to the loss of friction between the tire tread and the road, in these conditions, a speed indicator entirely dependent on the speed of rotation of the wheels would indicate a speed of zero when the vehicle is skidding on a sheet of ice or would remain at the speed before braking, during aquaplaning.
Such a particularly dangerous situation can only be taken into account by the driving assistance systems in question if the information collected is truly representative of the actual situation.
Furthermore, conventional speed sensors commonly used today, which measure the number of rotations of the wheel of the vehicle made during a specific time period can lead to inaccurate speed measurements if there are variations in wheel diameter due for instance to poor tire inflation in the case of automobiles, or to wearing of the wheel in the case of trains, or if the vehicle is skidding on the ground.
Various methods have been suggested to overcome such a problem and to allow a measurement of absolute ground speed independent of the speed of wheel rotation to be made by using the Doppler effect.
In such methods a Doppler effect radar comprises an antenna which transmits an acoustic or electromagnetic wave towards the ground and uses the frequency shift between the frequency of said wave and that of the wave reflected by a ground element or a surface irregularity situated in the zone scanned by the radar beam, the element or surface irregularity hereafter defined as a reflecting obstacle.
This frequency shift, hereafter called the Doppler frequency and designated fd, which results from the relative movement between the antenna and the reflecting obstacle is proportional to the speed of the moving body relative to the ground and to the cosine of the angle xcex1 defined as the angle between the direction of the wave at the reflecting obstacle, and the ground according to the equation:       f    d    =                    2        ⁢        v        ⁢                  xe2x80x83                ⁢                  cos          ⁡                      (            α            )                              c        ⁢          f      0      
where f0 is the transmitter frequency, v the speed of movement of the vehicle and c the speed of the wave, which leads to:   v  =            cf      d              2      ⁢              cos        ⁡                  (          α          )                    ⁢              f        0            
Because the accuracy on the speed depends directly on the accuracy of the angle xcex1, Doppler effect radars use highly directional antennae having a radar beam with a small aperture angle, so that the main part of the radiation transmitted and received by the antenna is centered along an angle xcex2 with the ground, the angles xcex1 and xcex2 being equal in such situations.
A drawback of this solution arises from unexpected variations in angle xcex2 resulting from a change in tilt of the vehicle, due for example to a modification of the load.
In order to remedy this drawback, U.S. Pat. No. 4,107,680 suggests using at least two antennae, one pointed in the direction of travel and the other pointed in the opposite direction in order to compensate for the variation in tilt.
A second serious drawback of this prior art originates from the fact that when using a narrow radar beam, a reflected wave is only generated if a reflecting obstacle is encountered on the small surface of ground scanned by the beam, this condition not being necessarily fulfilled when the ground is smooth for example, as applies in the presence of ice.
In order to remedy this second drawback, French patent nxc2x0 2 722 301 suggests instead of using a highly directional antenna, using an antenna with a wide aperture angle to increase the probability of the presence of reflecting obstacles on the ground, in the beam.
Nevertheless in this case, the angle xcex1 corresponding to the reflecting obstacle which reflects the wave towards the antenna constitutes an additional unknown variable.
In the said method, and in order to measure this angle and the speed, the sensor transmits simultaneously or non-simultaneously, two waves, one of fixed frequency and the other of varying frequency, the measurement of speed being made by identifying the Doppler frequencies for each of the above transmissions, produced by reflection on the same reflecting obstacle.
This method which brings a large improvement to apparatuses of this kind, requires however, sensitive instrumentation with large signal processing capacities.
The present invention which is particularly aimed at remedying these drawbacks, allows one to benefit from the advantages associated with the use of antennae with a wide aperture angle, even though it only requires simple instrumentation and simple methods of signal processing.
The present invention relates to the method of measuring the speed v of a moving object traveling in a direction parallel to the ground, the measurement being made by means of a Doppler radar with transmitter and receiver antennae fixed to the moving object at a certain height h above the ground and designed to transmit a radar beam towards the ground along a mean axis extending forwards or backwards relative to the direction of movement, said method including the following steps:
generating an electrical signal at a certain frequency by means of an oscillator,
from said signal and possibly after amplification, transmitting an incident wave towards the ground, by means of the transmitter antenna having a wide aperture angle in a vertical plane,
receiving a reflected wave, generated by reflection of the incident wave by a reflecting obstacle on the ground, by means of the receiver antenna with a wide aperture angle in the vertical plane,
mixing together part of the electrical signal supplied by the oscillator and the received signal, possibly after amplification thereby generating two signals, one signal at a frequency equal to the sum and the other at a frequency equal to the difference of the two signal frequencies entering the mixer,
filtering the signal from the mixer to generate a filtered signal proportional to the signal at a frequency equal to the frequency difference,
amplifying the filtered signal at a frequency equal to the frequency difference to generate a signal called Doppler signal,
looking for the different Doppler frequencies present in the Doppler signal, at close successive instants,
said method being essentially characterized in that it further includes the following steps
identifying in each Doppler signal obtained, Doppler frequencies at close successive instants corresponding to reflecting objects on the ground located in the zone scanned by the wave transmitted by the transmitter antenna, called identified Doppler frequencies,
calculating the theoretical evolution function representing the evolution as a function of time, of the Doppler frequency corresponding to a reflecting object, for a given speed, height of the transmitter and receiver antennae above the road and position of the reflecting obstacle,
selecting from the identified Doppler frequencies, those which correspond to the same reflecting obstacle at different successive instants using them to determine the speed of the moving body.
In preferred implementations of the method of the invention, use is also made of one or more of the following dispositions:
when the identification of the different Doppler frequencies is made using a Fourier transform method in order to determine the corresponding spectra, called Doppler spectra, the frequencies corresponding to the spectrum peaks in the Doppler spectra are searched for,
when the identification of the different Doppler frequencies is made using a Fourier transform method in order to determine the corresponding spectra, called Doppler spectra, the Doppler spectrum is decomposed into a sum of elementary spectra corresponding to the reflecting obstacles,
when the identification of the different Doppler frequencies is made using a Fourier transform method in order to determine the corresponding spectra, called Doppler spectra, a deconvolution of the obtained Doppler spectra is performed, in order to identify the Doppler frequencies corresponding to the reflecting obstacles,
when the identification of the different Doppler frequencies is made using the determination of the crossovers of the Doppler signal in the time domain, the Doppler frequencies corresponding to the reflecting obstacles in the radar beam are identified by comparison with those measured at previous instants,
when the identification of the different Doppler frequencies is made using a decomposition of the Doppler signal into a set of elementary time-varying responses corresponding to the reflecting obstacles, one looks among these elementary responses for those with an amplitude greater than the level of noise,
one selects, among the identified Doppler frequencies, those corresponding to the same obstacle by:
associating, from among the identified Doppler frequencies, the series of those which at successive instants, are representative of the same reflecting obstacles seen at their positions at these instants,
adjusting, by varying the parameters of speed, and position of reflecting obstacles at a given instant, the theoretical evolution function to the evolution as a function of time, of each series of points previously defined, corresponding to each of the obstacles present in the radar beam, the final speed retained being that corresponding to the best fit,
one selects, among the identified Doppler frequencies, those corresponding to the same obstacle, by performing a correlation between the Doppler frequencies identified at different successive instants and families of theoretical evolution functions in which the parameters of speed and position of the obstacle at a given instant are varied, the parameters for which one obtains the best correlation between the two being considered as the result of the measurement,
the same antenna is used for transmission and reception,
when the transmitter antenna is also the receiver antenna, the theoretical evolution function of the Doppler frequency corresponding to an obstacle, as a function of time is determined by applying the following equation:             f      d        ⁡          (      t      )        =            2      ⁢              f        0            ⁢      v              c      ⁢                        1          +                                    h              2                                                      (                                  x                  -                                      v                    ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                            2                                          
where f0 is the frequency of transmission, v is the speed of the vehicle relative to the obstacle and considered to be positive when said vehicle is approaching the obstacle, c the speed of propagation of the wave, x the position of the reflecting obstacle at a given instant measured from the projection on the ground of the position of the antenna, the angle xcex1 and x being related by the equation:             tan      ⁡              (        α        )              =          h      x        ,
the height h of the antenna above the road is measured by any known sensor,
the height h is measured by:
associating, from among the identified Doppler frequencies, the series of those which at successive instants, are representative of the same reflecting objects seen at their new positions at these instants,
adjusting, by varying the parameters of speed and position of reflecting obstacles at a given instant, and height of the antenna above the ground, the theoretical evolution function of the evolution as a function of time, of each series of points previously defined, corresponding to each of the obstacles present in the radar beam, the final height retained being that corresponding to the best fit,
the height h is measured by performing a correlation between the Doppler frequencies identified at different successive instants and families of theoretical evolution functions in which the parameters of speed, position of the obstacle at a given instant and height of the antenna above ground are varied, the parameters for which one obtains the best correlation between the two, being considered as the result of the measurement,
the transmitted wave is an electromagnetic wave,
the frequency of the transmitted electromagnetic wave is in the range 8 GHz-80 GHz and preferably in the range 20 GHz-80GHz,
the transmitted wave is an acoustic wave,
the frequency of the transmitted acoustic wave is in the range 20 kHz-500 kHz and preferably in the range 30 KHz-200 kHz,
when the frequency content of the calculated Doppler signal indicates the presence of a large number of reflecting obstacles in the radar beam, the Doppler frequency corresponding to the direction of maximum radiation from the transmitter antenna is selected and the speed is determined by applying the formula:   v  =            cf      d              2      ⁢              cos        ⁡                  (          β          )                    ⁢              f        0            
where xcex2 is the angle between the direction of maximum radiation from the transmitter antenna and the ground,
when the frequency content of the calculated Doppler signal indicates the presence of a large number of reflecting obstacles in the radar beam, the aperture angle of the transmission is reduced,
the mean speed obtained during different successive measurements is calculated.
The invention also provides an apparatus for implementing a method as defined above, the apparatus comprising:
an oscillator which supplies an electrical signal at a certain frequency,
a transmitter antenna having a wide angle of aperture in a vertical plane, which transmits from said first signal or possibly from said first signal amplified, an incident wave towards the ground,
a receiver antenna having a wide aperture angle in a vertical plane which receives a reflected wave generated by reflection of the incident wave on a reflecting obstacle on the ground,
a mixer circuit which takes part of the electrical signal supplied by the oscillator and mixes it with the signal received by the receiver, possibly after amplification, thereby generating two signals, one with a frequency equal to the sum of the two input frequencies and the other with a frequency equal to the difference between the two frequencies at the input of the mixing circuit,
a low pass filter which filters the output signal from the mixer circuit to generate a filtered signal proportional to the signal with frequency equal to the frequency difference,
a low frequency amplifier which amplifies said filtered signal thereby generating a signal called the Doppler signal,
means of identifying Doppler frequencies called identified Doppler frequencies in each Doppler signal obtained at close successive instants, which correspond to reflecting obstacles on the ground located in the zone scanned by the transmitted wave,
means of measuring the height of the transmitter and receiver antennae above the ground,
means of calculating the theoretical evolution function representative of the evolution as a function of time of the Doppler frequency corresponding to an obstacle, for a given speed, a given height of the transmitter and receiver antennae above the ground and for a given position of the obstacle,
means of selecting from among the identified Doppler frequencies those frequencies corresponding to the same obstacle at different successive instants and deducting from these the speed of the moving object.
In the preferred embodiment of the apparatus of the invention, use is also made of one or more of the following dispositions:
when the identification of the different Doppler frequencies is made using a Fourier transform method in order to determine the corresponding spectra, called Doppler spectra, the identification methods look for frequencies corresponding to the spectrum peaks in the Doppler spectra,
when the identification of the different Doppler frequencies is made using a Fourier transform method in order to determine the corresponding spectra, called Doppler spectra, a deconvolution of the obtained Doppler spectra is performed, in order to identify the Doppler frequencies corresponding to the reflecting obstacles,
when the identification of the different Doppler frequencies is made using the determination of the crossovers of the Doppler signal in the time domain, the Doppler frequencies corresponding to the reflecting obstacles in the radar beam are identified by comparison with those measured at previous instants,
when the identification of the different Doppler frequencies is made using a decomposition of the Doppler signal into a set of elementary time-varying responses corresponding to the reflecting obstacles, one looks among the elementary responses, for those with an amplitude greater than the noise level,
the selection methods select from among the identified Doppler frequencies those corresponding to the same obstacle by:
associating, from among the identified Doppler frequencies, the series of those which at successive instants, are representative of the same reflecting objects seen at their new positions at these instants,
adjusting, by varying the parameters of speed and position of reflecting obstacles at a given instant, and height of the antenna above the ground, the theoretical evolution function of the evolution as a function of time, of each series of points previously defined, corresponding to each of the obstacles present in the radar beam, the final height retained being that corresponding to the best fit,
the selection methods select from among the identified Doppler frequencies, those frequencies corresponding to the same obstacle by performing a correlation between the Doppler frequencies identified at different successive instants and families of theoretical evolution functions in which the parameters of speed, position of the obstacle at a given instant and height of the antenna above ground are varied, the parameters for which one obtains the best correlation between the two being considered as the result of the measurement,
the same antenna is used for transmission and reception,
when the transmitter antenna is also the receiver antenna, the means of calculating determine the theoretical evolution function as a function of time, of the Doppler frequency corresponding to an obstacle, by applying the following equation:             f      d        ⁡          (      t      )        =            2      ⁢              f        0            ⁢      v              c      ⁢                        1          +                                    h              2                                                      (                                  x                  -                                      v                    ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                            2                                          
where f0 is the frequency of transmission, v is the speed of the vehicle relative to the obstacle, considered to be positive when said vehicle is approaching the obstacle, c the speed of propagation of the wave, x the position of the reflecting obstacle at a given instant, measured from the projection on the ground of the position of the antenna, the angle xcex1 and x being related by the equation:       tan    ⁡          (      α      )        =      h    x  
means of measuring the height h of the antenna above the road use any known sensor.
means of measuring the height h operate by:
associating, from among the Doppler frequencies identified, the series of those which at successive instants, are representative of the same reflecting objects seen at their new positions at these instants,
adjusting, by varying the parameters of speed and position of reflecting obstacles at a given instant, and height of the antenna above the ground, the theoretical evolution function for the evolution, as a function of time, of each series of points previously defined, corresponding to each of the obstacles present in the radar beam, the final height retained being that corresponding to the best fit.
means of measuring the height h perform a correlation between the Doppler frequencies identified at different successive instants and, the families of theoretical evolution functions in which the parameters of speed, position of the obstacle at a given instant and height of the antenna above the ground are varied, the parameters for which the best correlation is obtained being considered as the result of the measurement,
the transmitted wave is an electromagnetic wave
the frequency of the transmitted electromagnetic wave is in the range 8 GHz-80GHz and preferably in the range 20 GHz-80 GHz,
the wave transmitted is an acoustic wave,
the frequency of the acoustic wave transmitted is in the range 20 kHz-500 kHz and preferably in the range 30 kHz-200 Khz,
when the frequency content of the calculated Doppler signal indicates the presence of a large number of reflecting obstacles in the radar beam, methods are foreseen to select those frequencies among the Doppler frequencies which correspond to the direction of maximum radiation from the transmitter antenna, the speed is determined by applying the formula:   v  =            cf      d              2      ⁢              cos        ⁡                  (          β          )                    ⁢              f        0            
where xcex2 is the angle between the maximum direction of radiation from the transmitter antenna and the ground,
when the frequency content of the calculated Doppler signal indicates the presence of a large number of reflecting obstacles in the radar beam, methods are foreseen to reduce the aperture angle of the antenna,
methods are foreseen to calculate the mean speed obtained during the different successive measurements.